The invention relates to a colour television display apparatus having a beam current reference level control circuit comprising a measuring circuit included in the cathode circuits of the electron guns of a multi-gun colour display tube for measuring during a measuring period the beam current reference level to be corrected, a level insertion circuit for providing during said measuring period a reference level in a video signal to be applied to each gun of the display tube, and a beam current level correction circuit coupled to the measuring circuit and to a control electrode of each gun of the display tube for obtaining substantially the same beam current reference level in each of the electron guns.
The Dutch Patent Application No. 7202401 (PHN 6130) discloses a colour television display apparatus of the above-mentioned type wherein by the use of a sequence circuit a great portion of the beam current reference level control circuits is in common for the different electron guns of the picture display tube. Then the beam current reference levels of these guns cannot substantially differ from one another. The colour rendition of this apparatus is very constant and substantially independent of disturbing influences. However, in general, the phosphors of the picture display tubes do not have the same efficiency so that the colour rendition for low luminance values is not quite correct if this rendition is correctly adjusted for high luminance values. It would be possible to obtain a correct rendition for low luminance values by using a beam current reference level which would be much lower than the presently used value of some .mu.A. This has practical disadvantages. In addition it would, for example, be possible to perform a correction by means of a circuit with which the beam current reference levels could be adjusted in a ratio matched to the efficiency of the phosphors. This would not only require an adjustment for high luminances but also an additional adjustment at a low luminance.